I've been doing some routing around.
Apparently, and surprisingly, no MOT certificate does NOT invalidate your insurance,
unless your policy specifically mentions it.
Most, allegedly, only say the car must be roadworthy.
No MOT doesn't necessarily mean it's not roadworthy though, it just shows you haven't bothered getting it tested!
However, lack of an MOT can become an issue if an item that would be tested during the MOT (steering/brakes etc.) is found to have contributed to an accident.
Clear as mud then.